P1.3 - Revision Energy – Transfer – Power - Efficiency

To be able to define and calculate efficiency. To be able to define power and calculate energy use. Learning Objectives

Starter Match up the definitions to the correct keyword: ENERGY POWER JOULES EFFICIENCY The standard measurement that we use to measure all types of energy. How good an appliance is at converting input energy into useful output energy with minimal waste energy. The name given to the amount of energy required by an appliance to do work. Measured in Watts (W). Something that absolutely everything has and can be transferred to other forms.

If this is the answer… Useful energy out divided by the the total amount of input energy. What is the formula for calculating efficiency of a device or appliance?

Efficiency Efficiency is the name given to something or someone that gets stuff done! Efficiency is also a measurement of how well something is done. In Physics; efficiency is concerned with how well a device or appliance does it’s job. As all Physicists are lazy, they don’t like using much energy to get stuff done.

Who is more efficient? I barely move during the day except for eating grass and going to the toilet, even then I just go where I’m standing. I don’t play sports either. I go for a run twice a day and chase the postman every morning. I can often be found chasing wasps and old ladies. I eat two small meals a day and I’m always up for playing fetch.

The Cow! The cow is most efficient because she uses less energy to get all her jobs done. She doesn’t even waste energy by moving away from where she eats to go to the toilet. Most of the energy that the cow consumes goes into milk and meat production: THIS IS EFFICIENCY!

How do we calculate efficiency? Simple formula: Efficiency = Useful Energy OUT Total Energy IN X 100 = % EXAM TIP: Always show your workings and never forget to add the Joules!

Your Turn!

Defining Power Power is defined as: ‘The rate at which energy is transferred’ Power is measured in WATTS (W)

Calculating Energy Uses We can work out how much energy a device or an appliance uses if we know WATT? Energy (J) = Power (Watt) X Time (Seconds)

Your Turn…

Plenary 1)Work out which animal is most efficient and explain why. 2)Create a rhyme/rap or way of remembering how to calculate energy usage…

Homework Find as many power labels (find Watt rating) as you can in the home and work out how much energy they use in 20 minutes.

Calculating Energy Costs Every quarter (3months) your electricity supplier will send you a bill telling you how much electricity you have used. Do you actually use up electricity? NOPE!NOPE! We pay for the work done by the electricity that comes to our house.

KWh What does KWh stand for? KWhKWh Kilo = 1000 Power (W) Hour

How do we calculate the cost? Wattage x hours used ÷ 1000 x price per kWh = cost of electricity. Hours (not seconds). Power rating (W) What your energy company charges. The average price per KWh is 8p

Calculating Costs Wattage x hours used ÷ 1000 x price per kWh = cost of electricity. 40W Lamp used for 12 hours at 8p per KWh 40 x 12 = / 1000 = 0.48 x 0.08p = 0.03p (3pence) 40W Lamp used for 12 hours at 8p per KWh 40 x 12 = / 1000 = 0.48 x 0.08p = 0.03p (3pence)

Your Turn 1)A 200W Computer used for 4 hours. 2)A 60W TV used for 6 hours. 3)A 2000W Electric oven for 1 hour. 4)Charging a mobile phone at 0.5W for 5 hours. 5)A 1400W Microwave for 1 hour. Wattage x hours used ÷ 1000 x price per kWh = cost of electricity.

1) 6p 2) 2p 3) 16p 4) 0p effectively 5) 11p

Watt, no Watts? If your device doesn't list wattage, but it does list amps, then just multiply the amps times the voltage to get the watts. For example: 2.5 amps x 240 volts = 600 watts Power (W) = Amperes (A) x Voltage (V) UK voltage is 240V Power (W) = Amperes (A) x Voltage (V) UK voltage is 240V

Where do you find the Amps? Usually stamped on the outside of the plug but open it up and look at the fuse itself.

Homework Calculate the energy used by at least five devices in the home. Calculate the cost of the devices you use at home in a day. Due: Monday 1 st October